The invention relates to polyethylene blends. The blends comprise a high molecular weight, high density polyethylene (HMW HDPE) and a linear low density polyethylene (LLDPE) that is a copolymer of ethylene with hexene. The invention also relates to geomembranes and medium density pipes made from the blends.
Polyethylene is divided into high density (HDPE, density 0.941 g/cc or greater), medium density (MDPE, density from 0.926 to 0.940 g/cc), low density (LDPE, density from 0.910 to 0.925 g/cc) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.) Polyethylene is useful in many commercial applications, however each application requires unique properties designed for the particular needs of the application.
For applications such as medium density pipes and geomembranes, it is important that the polyethylene meet high Environmental Stress Cracking Resistance (ESCR). Stress cracking according to ASTM D883 is xe2x80x9can external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength.xe2x80x9d Environmental stress cracking typically occurs in the presence of surface wetting agents such as alcohols, soaps, surfactants, and others. Thus, high resistance to environmental stress cracking is a particularly important performance criteria for polyethylene to be used in medium density pipe and geomembrane applications. ESCR is measured by a variety of different analytical methods, including the bent strip ESCR test (ASTM D1693) and the notched constant tensile load (NCTL) test (ASTM D5397). Typically, geomembranes, for instance, require a minimum notched constant tensile load, as measured by ASTM D5397, of greater than 200 hours. However, industry requirements continue to push the NCTL minimum to higher numbers, thus requiring improvements in the polyethylene and polyethylene blends used in these applications.
U.S. Pat. No. 5,338,589 discloses a polyethylene molding composition consisting of 50 to 80% of a high density polyethylene having a broad bimodal molecular weight distribution and 20 to 50% of a linear low density polyethylene. The disclosed composition is described as possessing good resistance to environmental stress cracking and is useful in MDPE pipes. As with any polymer composition, it is advantageous to improve properties including ESCR for use in applications such as MDPE pipes and geomembranes.
In sum, improved polyethylene blends with higher resistance to environmental stress cracking are needed. These polyethylene blends would be extremely useful in applications such as geomembranes and MDPE pipes.
The invention is a blend comprising a high molecular weight, high density polyethylene (HMW HDPE) and a linear low density polyethylene (LLDPE). The blend comprises from about 50 wt. % to about 80 wt. % of HMW HDPE having a density from about 0.94 g/cc or greater and a melt index MI2 of less than about 0.1 dg/min. The blend also comprises about 20 wt. % to about 50 wt. % of LLDPE, where the LLDPE is a copolymer of ethylene with hexene. The LLDPE has a density within the range of about 0.90 to about 0.93 g/cc and an MI2 within the range of about 0.50 to about 5 dg/min. The LLDPE is a high performance LLDPE that has a dart drop impact strength of greater than about 500 grams as measured by ASTM D1709/A. I have surprisingly found that blending the HMW HDPE and the LLDPE gives enhanced environmental stress cracking resistance compared to blends using standard LLDPEs.